1. Field of the Invention
This invention relates to testing printed circuits, and more particularly, to testing circuits extending along both sides of a flexible substrate for shorts and open conditions.
2. Background Information
Test processes used for printed circuit boards include a two-probe method, in which probes are simultaneously placed at two points, or nodes, on a circuit board to measure the electrical characteristics, such as resistance and continuity, of the portion of the circuit extending between these points, and a single-probe method, in which a probe is placed at a single point on the circuit board to measure the capacitance between a portion of the circuit extending from this point and a conductive surface separated from the circuit by dielectric material. This conductive surface may be a part of a circuit board being tested, such as a ground or voltage plane in a multi-layer circuit board, or it may be a conductive surface to which the circuit is held during the test process.
Circuit board testing is typically concerned with detecting the presence of electrical "shorts" or "opens." A shod occurs when two adjacent circuits, which should be separated, are inadvertently electrically connected instead. An open occurs when there is a break in a circuit line extending between two points which should be electrically connected. Both the two-probe method and the single-probe method can be used to locate both shorts and opens. However, while the two probe method is adept at finding opens by simultaneously probing the two points which should connected, the use of this method to find shorts is particularly complex and time consuming. With this method, while a first probe is held on a first point, a second probe must be moved among all of the other test points connected to circuit traces which are at some point adjacent to the circuit extending from the first point. With the single-probe method, the capacitance between each test point and the conductive plane is checked. If this capacitance is below an expected minimum, it is determined that an open has apparently occurred. If this capacitance is above an expected maximum, it is determined that a short has apparently occurred. Thus, the single-probe method greatly reduces the number of tests otherwise required to adequately check for shorts using the two-probe method.
Circuit test processes of these types may be performed on flexible substrates, as well as on rigid printed circuit boards. Flexible substrates include individual layers, which are eventually laminated together to form rigid multi-layer circuit boards, and flexible circuits of various types, which are increasingly used to provide circuits that can be formed into various shapes and to provide circuits that can be changed in shape during their operation. Many of these flexible materials are supplied and tested in a reel to reel form. In general, flexible materials must be supported through the test process. A backing support on the side of the materials opposite the side to which the probes are applied is particularly important, since the mechanical forces applied to the circuit by the probes must be resisted. Also, it is desirable to hold the flexible material flat during the test process, or to otherwise control its geometry relative to the locations of the probes. Furthermore, if the single-probe method is to be used with a flexible circuit not having a conductive plane, a backing plate must provide the conductive plane to which circuit capacitance is measured.
In a number of circuit applications, circuit traces are provided on both sides of flexible material. In such applications, it is particularly desirable to test both sides of the circuit simultaneously, to improve the throughput of a test process which would otherwise require two passes and to minimize damage to the circuits which might otherwise occur during handling through two passes. Circuits on the opposite sides of both circuit boards and flexible substrates are often connected by vias extending through the insulating substrate material, so that an individual circuit line extends partly along each side of the insulating material,
What is needed is a method to provide for simultaneously testing circuits on both sides of either a rigid circuit board or a flexible substrate, using either the two-probe or single probe method. This method should provide physical support to the circuit material during the engagement of various probes, while providing a conductive backing layer as required by the single-probe method, and while providing clearance for the movement of probes to engage various circuit points.